Machinists' Decision Will Help Determine Assembly Site for 777X

Boeing Co.'s largest union is set to vote Friday on a contract that will likely shape the power of organized labor at one of the U.S.'s biggest manufacturers for years.

If members of the International Association of Machinists and Aerospace Workers, which represents more than 32,000 Boeing employees, approve Boeing's eight-year contract offer, the aerospace company has vowed to assemble its planned 777X and manufacture its...